The invention relates to the preparation of polyolefin foams, particularly polystyrene foams, by extrusion of a heat plasticized polyolefin resin in the presence of an environmentally acceptable blowing agent or blowing agent blend.
It is well known in the art to prepare a variety of polyolefin foams by blending molten or heat plasticized polyolefin resin with a volatile foaming agent or blowing agent, and extruding the resin/foaming agent mixture from a pressurized chamber through an orifice into a zone of lower pressure. Upon release of the pressure the normally gaseous volatile foaming agent vaporizes and expands the polyolefin to form a stable cellular body.
A variety of volatile blowing agents have been employed in preparation of such foams. Pentane has been proven useful particularly in connection with the blowing of polystyrene foams. However, n-pentane is highly flammable and for this reason elaborate and expensive precautions must be taken when it is used.
Various volatile halogenated hydrocarbons that are non-flammable or barely flammable have been tried as blowing agents for polyolefin foams. However, most of the halogenated hydrocarbons tested failed to produce foam with good properties. Methylene chloride, trichlorofluoromethane and chlorodifluoromethane are examples. Methylene chloride and trifluoromethane are too soluble in the polystyrene and result in foam which has a non-uniform and excessively large cell size. While chlorodifluoromethane produces a foam with acceptable structural properties, the residual chlorodifluoromethane contained in the product restricts its handling in certain applications, especially as food containers. FDA has specified that chlorodifluoromethane levels must be below certain values before direct food contact (see Food Drug and Cosmetic Act section 409(b)(5)(21 U.S.C. 348(b)(5)).
U.S. Pat. No. 4,228,246 discloses the use of dichlorodifluoromethane as a blowing agent for the preparation of a variety of polyolefin foams, including polyethylene and polystyrene. However, particularly in the case of polystyrene, dichlorodifluoromethane has proven to be unduly insoluble, resulting in premature foaming which yields foam with excessively small cell size. Furthermore, the rapid foaming and the poor solubility characteristics of dichlorodifluoromethane cools the product too soon after the extrusion which then requires excessive "cold working" i.e. shaping the resulting foam after extrusion at temperatures near or below its solidification temperature. Further, large amounts of energy must be used to process polystyrene foam containing dichlorodifluoromethane as the blowing agent because extrusion processability is poor. The blowing agent solubility requirements for producing foams with good properties are reviewed by J.G. Burt, Proc. of Soc. of Plastics Engineers, p. 31-34, Quebec (Apr. 1977).
U.S. Pat. No. 3,539,473 discloses the use of certain blends of iso- or n-pentane and dichlorodifluoromethane as blowing agents for certain polyolefin foams.
A disadvantage of continuing to use dichlorodifluoromethane is that fully and even partially halogenated chlorinated hydrocarbons have been implicated in causing environmental problems. Specifically, CFC-12 (dichlorodifluoromethane) has substantial potential for stratospheric ozone depletion and global warming. Chlorodifluoromethane has become widely used because of its lower ozone depletion than dichlorodifluoromethane but it does have disadvantages. It has an ozone depletion potential and the need for low residuals requires slowed processing or warehousing. Further hydrocarbons like pentane are volatile organic compounds (VOC) that can cause smog. VOC emissions and the use of CFCs are being regulated.
It is apparent that there is a need in this art for novel volatile blowing agents or blowing agent blends which are capable of efficiently producing polyolefin foams having good properties, good extrusion processability and no or minimal negative environmental impact.
Further, because foam containers made from blowing agents which have not been approved for food contact and which have low permeabilities are required to stand before receiving food, blowing agents which have higher permeabilities, which would decrease the required standing time of the foam containers are desirable.